Implement Worker class:

* Move recipe processing code in the worker;
* Refactor CLI and use the the worker;
* Implement Recipe#tasks and remove tasks application during evaluation,
  tasks are now applied by the worker after all evaluations are done.

lib/producer/core/recipe.rb

@@ -1,7 +1,7 @@
 module Producer
   module Core
     class Recipe
-      attr_reader :code, :filepath
+      attr_reader :code, :filepath, :tasks
 
       def self.from_file(filepath)
         new(File.read(filepath), filepath)
@@ -10,11 +10,13 @@ module Producer
       def initialize(code, filepath = nil)
         @code     = code
         @filepath = filepath
+        @tasks    = []
       end
 
       def evaluate(env)
         dsl = DSL.new(@code).evaluate(env)
         dsl.tasks.map { |e| e.evaluate env }
+        @tasks = dsl.tasks
       end
     end
   end